1. Technical Field
The present invention relates to the technical field of a charge roller, a method for producing the charge roller, and an electrophotographic apparatus including the charge roller, such as an electrostatic copier, a printer, or a facsimile. The charge roller includes gap members formed of tape-shaped films fixed to the roller by winding them around both ends of the roller to define a predetermined charge gap for noncontact charging between the roller and an image-bearing member.
2. Related Art
JP-A-2001-296723, for example, discloses an image-forming apparatus that defines a predetermined charge gap between a charge roller and an image-bearing member to charge the image-bearing member without contact. Referring to FIG. 14A, a charge roller 14a included in the image-forming apparatus disclosed in this publication includes a core 14b, a resistive layer 14c disposed on the circumferential surface of the core 14b and formed of a conductive elastic material, and a pair of annular gap members 14d and 14e fixed to the resistive layer 14c by winding them around both ends of the circumferential surface of the resistive layer 14c and formed of insulating tape-shaped films. A predetermined charge gap G is defined by bringing the pair of gap members 14d and 14e into contact with the circumferential surface of a photosensitive drum 14f serving as an image-bearing member. Rotating shafts 14g and 14h protrude coaxially from end surfaces of the core 14b in the axial direction thereof. The rotating shafts 14g and 14h are supported by bearings 14i and 14j, respectively. Compression springs 14k and 14m urge the bearings 14i and 14j, respectively, against the photosensitive drum 14f to bring the gap members 14d and 14e into contact with the circumferential surface of the photosensitive drum 14f. 
When the charge roller 14a charges the photosensitive drum 14f without contact across the charge gap G, the charge gap G can prevent generation of ozone, adhesion of foreign matter such as toner from the photosensitive drum 14f to the charge roller 14a, and adhesion of substances contained in the resistive layer 14c to the photosensitive drum 14f. The charge roller 14a can thus charge the photosensitive drum 14f with higher charge performance.
When a tape-shaped film is wound around a charge roller, a seam is left between both ends of the film in the circumferential direction. The charge roller must be rotated with a uniform charge gap maintained at any position in the circumferential direction so that the roller can constantly perform stable, excellent charging. To meet the requirement, the film must be wound around the charge roller with no gap or overlapping portion (overlapping in the radial direction of the charge roller 14a) left at the seam between both ends of the film (in the circumferential direction of the charge roller). To wind the film around the charge roller in such a manner, however, the length of the film must be defined with high accuracy, and the film must also be wound around the charge roller with high accuracy. Consequently, the dimensions of the film must be extremely strictly controlled, and thus low productivity and high cost result.
If the dimensional accuracy and winding accuracy of the film are lowered for improved productivity and cost reduction, a gap or an overlapping portion inevitably occurs at the seam between both ends of a gap member formed by winding the film around the charge roller by substantially one turn. Such a condition can result in the absence of the gap member at the seam or a variation in the thickness thereof, and thus the charge gap can vary when the seam reaches the nip (contact position) between the gap member and an image-bearing member. In that case, the charge roller fails to constantly perform stable, excellent charging.
For the charge roller 14a disclosed in the publication above, as shown in FIGS. 14B and 14C, both ends 14d1 and 14d2 of the film of the gap member 14d are cut obliquely, and the film is wound around the charge roller 14a with a gap 14s left at the seam between the ends 14d1 and 14d2. The film can thus be wound around the charge roller 14a so that the gap member 14d extends in the axial direction at every position in the circumferential direction of the charge roller 14a. Even though the seam is left, accordingly, a constant charge gap G can be maintained without extremely strict control of the dimensions of the film. Although not illustrated, the film of the other gap member 14e is similar to the film of the gap member 14d. 
Referring to FIGS. 14D and 14E, the film of the gap member 14d may also be spirally wound around the charge roller 14a by a length that is larger than two turns by a predetermined length. The film is shifted for each turn in the axial direction of the charge roller 14a without overlapping in the radial direction thereof, and the end 14d2 is shifted from the end 14d1 in the axial direction. Thus, the gap member 14d extends in the axial direction at every position in the circumferential direction of the charge roller 14a. Such spiral winding leaves no seam between the ends 14d1 and 14d2 because the film is wound around the charge roller 14a by a length that is larger than two turns by a predetermined length such that the film is shifted in the axial direction for each turn. Accordingly, a constant charge gap G can be maintained without extremely strict control of the dimensions of the film. Although not illustrated, the film of the other gap member 14e is similar to the film of the gap member 14d. 
It should be noted that, although not described here, the publication above also discloses other methods for forming gap members extending in the axial direction at every position in the circumferential direction of the charge roller 14a. 
If, however, the film of the gap member 14d is simply wound around the circumferential surface of the charge roller 14a, the ends 14d1 and 14d2 of the film are exposed and come into contact with the photosensitive drum 14f either in the case of FIG. 14B or in the case of FIG. 14D. In FIGS. 14C and 14E, the ends 14d1 and 14d2 repeatedly come into contact with the photosensitive drum 14f when the charge roller 14a rotates clockwise. The photosensitive drum 14f then repeatedly exerts a force on the film at the nip between the charge roller 14a and the photosensitive drum 14f so as to peel the end of the film at which it starts entering the nip. This force undesirably peels the film off the charge roller 14a and curls it up. In particular, the gap member 14d can peel more frequently if the charge roller 14a is formed of an inelastic material.
In addition, foreign matter such as toner particles intrudes the gap 14s of the seam between the ends 14d1 and 14d2 of the film and adheres to the film during image-forming operation. Such foreign matter deteriorates the adhesion between the ends 14d1 and 14d2 of the film and the charge roller 14a, and thus promotes peeling of the film at the ends 14d1 and 14d2 thereof. The same holds true for the film of the other gap member 14e. 
If the ends of the gap members 14d and 14e are curled, the charge gap G varies when the charge roller 14a rotates, and thus the gap members 14d and 14e fail to maintain a constant charge gap G. This makes it difficult to perform uniform, stable, excellent charging of the photosensitive drum 14f. 
If the film is wound in layers to form the gap members 14d and 14e, the photosensitive drum 14f suffers from discharge during noncontact charging using a superimposed bias voltage containing DC and AC voltage components. The discharge undesirably decreases the thickness of a charged portion of the photosensitive drum 14f with increasing number of prints. The problem of decreased thickness makes it difficult to control the charge gap G to a predetermined width or less or successfully charge the photosensitive drum 14f. 